1. Field of the Invention
The invention relates to a process for preparing 1,2,3,6-tetrahydro-2,2,6,6-tetramethylpyridine (hereinafter referred to as THTMP) from 4-hydroxy-2,2,6,6-tetramethylpiperidine (hereinafter referred to as TAA-ol).
2. Description of the Related Art
THTMP is of considerable interest because the industrially important 1,2,3,6-tetrahydro-2,2,6,6-tetramethylpyridin-N-oxyl can be prepared from it. Furthermore, 2,2,6,6-tetramethylpiperidin-N-oxyl, known as TEMPO, has become readily obtainable by hydrogenation and subsequent oxidation. Both N-oxyls serve as polymerization inhibitors for monomers such as acrylic acid and its esters.
Syntheses of THTMP by dehydration of TAA-ol are known from the literature. E. Fischer was able to carry out this reaction in good yield using concentrated sulfuric acid (Chem. Ber. 16, 1604 (1883)). However, this requires a large excess of sulfuric acid which has to be neutralized after the reaction. Reaction of TAA-ol with concentrated hydrochloric acid at elevated temperatures under pressure gives THTMP in addition to the desired chloro compound, from which THTMP is likewise obtainable by dehydrohalogenation (Frankenhauser et al., Helv. Chim. Acta, 49, 690 (1966)).
These and other processes described in the literature (Samtleben, Chem. Ber. 32, 644 (1899)) have a high consumption of chemicals, are technically complicated and are associated with problems in waste disposal.
It is therefore an object of the invention to develop a process which can be carried out without consumption of chemicals and presents no problems in waste disposal.